abstract class Spacecraft {
  String get name;
  DateTime? get launchDate;

  void describe();
}

class MockSpaceship implements Spacecraft {
  @override
  String name = 'Mock Spaceship';

  @override
  DateTime? launchDate;

  @override
  void describe() {
    print('模拟航天器: $name');
    if (launchDate != null) {
      print('模拟发射日期: $launchDate');
    } else {
      print('未发射');
    }
  }
}

void main() {
  var mockShip = MockSpaceship();
  mockShip.describe();
}
